Apple Insider writes the code of Apple's website included a reference to an "iPhone Extreme":
Credit for this find goes out to Tom over at the iPhoneBugList, who stumbled upon the reference while attempting to interface his own site with Apple's iPhone feedback form (which is where the reference was spotted).
Nestled within the HTML code for the feedback form is a hidden variable called "product" which contain the value "iPhone Extreme." Unfortunately, that's the extent of the rumor.
Obviously, speculation is that "iPhone Extreme" could represent some new product down the line, a new name for the current iPhone if a cheaper and less capable version is released, or simply nothing at all.
On the other hand, this hidden variable with its associated value of "iPhone Extreme" is currently being submitted to Apple's feedback script each and every time a user fills out the form.